C. P. Lusher

Interfacial Friction of Submonolayer Helium Films Adsorbed on the Surface of Graphite

The frictional coupling between an adsorbed helium film and the surface of graphite has been studied using torsional oscillator methods and exfoliated graphite as a substrate, in the temperature range 50mK to 2K. The observed period shift is consistent with the film decoupling from the substrate as the temperature is reduced, and is accompanied by a maximum in dissipation. At coverages below registry and in the incommensurate solid region the data are well described by Debye relations with a thermally activated relaxation time. This would correspond to a vanishing of the interfacial friction for the solid monolayer as T -→ 0. At coverages between 0.064Å2 and 0.08Å-2 a double peaked structure in the dissipation is observed. The results relate to the work of Krim et al.1 who have used a quartz microbalance to study the “nanotribology” of adsorbed monolayers. In the present work the use of the graphite substrate offers the possibility of studying these effects on a system with a rich and well characterised submonolayer phase diagram.

Nuclear magnetic resonance on room temperature samples in nanotesla fields using a two-stage dc superconducting quantum interference device sensor

We describe a compact system for pulsed nuclear magnetic resonance at ultralow magnetic fields on small liquid samples (∼0.14ml) at room temperature. The broadband spectrometer employs an integrated two-stage superconducting quantum interference device current sensor with a coupled energy sensitivity of 50h, in the white noise limit. Environmental noise is screened using a compact arrangement of mu-metal and a superconducting shield. Proton signals in water have been observed down to 93nT (a Larmor frequency of 4.0Hz), with a minimum linewidth of 0.16Hz measured at ∼40Hz. Two-component free induction decays were observed from oil/water mixtures between 275 and 300K.

Torsion pendulum for the study of thin 3He films

We report on the design of a torsion pendulum that can resolve the mass loading from 5×10173He atoms (equivalent to a 1000Å film) with a 0.1% resolution. The oscillator is fabricated from coin silver alloy, and the working surfaces are two highly polished coin silver discs, each with well-characterized surface roughness, that are diffusion welded together using a copper gasket. We report on the cells temperature dependent background. The cell will be used to examine the evolution of the superfluid density and transition temperature as a function of film thickness as well as the normal fluid behavior.

Current-sensing noise thermometry from 4.2 K to below 1 mK using a DC SQUID preamplifier

Abstract We are using a low-Tc DC SQUID to perform current-sensing noise thermometry, by measuring the thermal noise currents in a copper resistor. The temperature is obtained from the Nyquist formula. This is a practical thermometer for use from 4.2 K to below 1 mK , with a percentage precision independent of temperature. Using a 0.34 mΩ resistor, the thermometer had an amplifier noise temperature TN of 8 μK . A precision of 1.5% was obtained in 200 s . The thermometer was in good agreement with the PLTS-2000 3 He melting curve scale down to 4.5 mK . We have cooled the thermometer successfully below 1 mK , achieving a minimum electron temperature of 300 μK .

A nuclear magnetic resonance spectrometer for operation around 1MHz with a sub-10-mK noise temperature, based on a two-stage dc superconducting quantum interference device sensor

We have developed a nuclear magnetic resonance spectrometer with a series tuned input circuit for measurements on samples at millikelvin temperatures based on an integrated two-stage superconducting quantum interference device current sensor, with an energy sensitivity e=26±1h when operated at 1.4K. To maximize the sensitivity, both the nuclear magnetic resonance spectrometer pickup coil and tuning capacitor need to be cooled, and the tank circuit parameters should be chosen to equalize the contributions from circulating current noise and voltage noise in the superconducting quantum interference device. A noise temperature TN=7±2mK was measured, at a frequency of 0.884MHz, with the circuit parameters close to optimum.

Primary current-sensing noise thermometry in the millikelvin regime.

The use of low-temperature platforms with base temperatures below 1 K is rapidly expanding, for fundamental science, sensitive instrumentation and new technologies of potentially significant commercial impact. Precise measurement of the thermodynamic temperature of these low-temperature platforms is crucial for their operation. In this paper, we describe a practical and user-friendly primary current-sensing noise thermometer (CSNT) for reliable and traceable thermometry and the dissemination of the new kelvin in this temperature regime. Design considerations of the thermometer are discussed, including the optimization of a thermometer for the temperature range to be measured, noise sources and thermalization. We show the procedure taken to make the thermometer primary and contributions to the uncertainty budget. With standard laboratory instrumentation, a relative uncertainty of 1.53% is obtainable. Initial comparison measurements between a primary CSNT and a superconducting reference device traceable to the PLTS-2000 (Provisional Low Temperature Scale of 2000) are presented between 66 and 208 mK, showing good agreement within the k=1 calculated uncertainty.

DC SQUID Spectrometers for NMR

We report experiments in which we have used DC SQUIDs in pulsed NMR spectrometers to observe directly the free precession of nuclear spins. A broadband spectrometer, which operates in flux-locked loop mode with a bandwidth of 3.4 MHz using a SQUID with additional positive feedback and an untuned superconducting input circuit, has been used to observe NMR signals from platinum powder at frequencies of 38, 65, 85, 240 and 513 kHz. The performance is compared with a second system in which a DC SQUID is operated open loop as a small signal rf amplifier with a series tank input circuit tuned at 1 MHz.

Low field DC SQUID nuclear magnetic resonance on single crystal UPt3

Abstract A SQUID spectrometer is being used to study high-quality single crystals of UPt 3 in low magnetic fields by performing pulsed NMR on 195 Pt. The system uses a multiloop DC SQUID with additional positive feedback and operates in flux-locked loop mode from DC to 3 MHz. It has an overall coupled energy sensitivity of 800 h and a dead time of ∼5 μs. NMR signals from UPt 3 have been observed in both the superconducting mixed state and in the normal state. A bulk platinum marker is used to determine the magnetic field. Measurements of 195 Pt Knight shifts in UPt 3 are reported.

A self-contained 3He melting curve thermometer for dissemination of the new provisional low-temperature scale

Abstract We report progress in the development of a self-contained 3 He melting curve thermometer, designed to be both easy to construct and simple to operate. It is based on a cylindrical pressure gauge, with good linearity of pressure versus inverse capacitance making calibration straightforward. The gas handling system is compact and in principle automatic. The readout electronics is based on a tunnel diode oscillator circuit, since one of the capacitance plates of the gauge is necessarily grounded. We present preliminary data on the performance of the thermometer, which will allow convenient dissemination the new provisional low-temperature scale PLTS-2000.

Low-frequency broadband NMR on UPt3 using DC SQUIDs

Abstract An NMR spectrometer has been constructed based on a DC SQUID with additional positive feedback, operating in flux locked loop mode. The system has an overall bandwidth of 3MHz and a coupled energy sensitivity of 800 h . The spectrometer is being used to study high-quality single-crystal samples of UPt 3 , in order to investigate the superconducting order parameter. NMR signals have been observed in the superconducting mixed state at significantly lower frequencies than hitherto, down to 100 kHz.